1. Field of the Invention
The present invention relates to an optical disc apparatus which performs information reproduction from an optical disc or performs information recording/reproduction on/from the optical disc, and more specifically to an optical disc apparatus having a function of reducing a crosstalk component of a trucking error signal appearing in a focus error signal.
2. Description of the Prior Art
Conventionally, the optical disc apparatus is provided with: an optical pickup which emits a laser beam for performing information reproduction from the optical disc or performing information recording/reproduction on/from the optical disc and also which receives a reflecting beam from the optical disc; and a servo control part which performs tracking servo and focus servo on the optical disc of this optical pickup. In such an optical disc apparatus, an RF amplifier which amplifies an RF signal read from the optical disc by the optical pickup is provided with a focus error signal generation circuit which generates a focus error signal.
FIG. 8 shows a block diagram of a focus error signal generation circuit included in the RF amplifier in the conventional optical disc apparatus. This focus error signal generation circuit 137a is included inside an IC (integrated circuit) composing the RF amplifier. The focus error signal generation circuit 137a is provided with: an input terminal VIN5 for inputting a detection signal a as an output signal of a detection region A of an optical detector 2b included in the optical pickup; an input terminal VIN6 for inputting a detection signal b as an output signal of a detection region B, an input terminal VIN7 for inputting a detection signal c as an output signal of a detection region C, and an input terminal VIN8 for inputting a detection signal d as an output signal of a detection region D.
The detection signal a inputted to the VIN5 is amplified by two-stage focus servo drive amplifiers a1 and a2, and then inputted to an adder 31. The detection signal b inputted to the VIN6 is amplified by two-stage focus servo drive amplifiers b1 and b2, and then inputted to an adder 32. The detection signal c inputted to the VIN7 is amplified by two-stage focus servo drive amplifiers c1 and c2, and then inputted to the adder 31. The detection signal d inputted to the VIN8 is amplified by two-stage focus servo drive amplifiers d1 and d2, and then inputted to the adder 32. The focus servo drive amplifiers a1 to d2 are adjusted at the same gain by a gain change processing circuit G2 controlled by a servo control part 104a. 
The adder 31 adds together an output signal of the focus servo drive amplifier a2 and an output signal of the focus servo drive amplifier c2. The adder 32 adds together an output signal of the focus servo drive amplifier b2 and an output signal of the focus servo drive amplifier d2. The subtracter 33 subtracts an output signal of the adder 32 from an output signal of the adder 31, and outputs this subtraction result as a focus error signal FE.
Hereinafter, a description will be given concerning the process performed, in the conventional optical disc apparatus having the focus error signal generation circuit 137a with the configuration as described above, from when a CD which produces more crosstalk than a DVD is inserted to when focus servo is turned on. The more crosstalk produced by the CD than by the DVD in the event of positional balance shift of the detection regions A, B, C, and D is possibly attributable to difference in the laser power and wavelength, or difference in the disc structure, or the like.
After inserted in the optical disc apparatus, the CD rotates, and recording information is read from the CD by the optical pickup. From the optical detector 2b of the optical pickup, an RF signal is outputted which is then amplified by the RF amplifier (not shown) and inputted to the servo control part 104a. 
After the CD is inserted in the optical disc apparatus and preparation for reading has been completed, during a processing period when the optical pickup irradiates the CD with a laser beam specific to a CD and then the optical disc determines whether or not the optical disc is a CD, the following processing is performed. Specifically, the servo control part 104a, before performing focus servo control, first measures the amplitude level of a focus error signal for performing focus search. In order to bring the measured amplitude level of the focus error signal a target value, the servo control part 104a sets the gains of the focus servo drive amplifiers d1 and d2 as initial values and adjusts the amplitude level of the focus error signal. If the gains of the focus servo drive amplifiers a1 to d1 at the first stage are previously fixed at the same value, the initial values of the gains of the focus servo drive amplifier a2 to d2 at the second stage are set.
Next, if it is determined that the inserted optical disc is a CD, during the processing period when the optical pickup irradiates the CD with a laser beam specific to a CD to perform focus search, the servo control part 104a measures the amplitude level of the focus error signal. Subsequently, based on the measured amplitude level of the focus error signal, the servo control part 104a performs S-curve balance adjustment of the focus error signal. Then, the servo control part 104a turns on focus servo control, thereby permitting the focus servo control to be performed on the CD of the optical pickup.
The optical pickup is provided with the optical detector 2b which receives a reflecting beam from the optical disc and which has four detection regions (for example, detection regions of four light receiving elements such as photodiodes or the like) A, B, C, and D arranged in a 2 by 2 matrix. However, an error (positional balance shift) in the fitting position of these light receiving elements causes a phenomenon that a component of a trucking error signal affects a focus error signal upon track jumping across a land and a groove on the optical disc, that is, T-F crosstalk (hereinafter simply referred to as crosstalk), thereby causing defocusing of a laser beam emitted from the optical pickup or resulting in difficulty in turning on focus servo.
In the focus error signal generation circuit 137a in this conventional optical disc apparatus, the gains of the focus servo drive amplifiers a1 to d2 for the detection signals a, b, c, and d from the detection regions A, B, C, and D are not individually adjusted. Thus, in the event of positional balance shift of the detection regions A, B, C, and D, this positional balance shift is directly amplified by the focus servo drive amplifiers d1 and d2, thus resulting in a larger amplitude level difference among the detection signals. This leads to failure to reduce the crosstalk, thus causing a seek failure on the optical disc of the optical pickup or a vibration phenomenon in the optical pickup, which in turn poses a problem of defocusing during seek.
JP-A-2002-32924 discloses an offset detector and an offset corrector of an optical head. In these devices, the levels of detected outputs a to d of four divided light receiving regions are each detected by a level detection circuit. Next, based on the levels of the detected outputs a to d, a gain adjustment circuit performs level adjustments of a first addition output of the detected outputs a and c, and a second addition output of the detected outputs b and d, and performs control so that the focus error signal, i.e., a difference between the first and second addition outputs becomes “0”. This conventional technology does not adjust the detected outputs a to d individually, thus resulting in failure to reduce the crosstalk that is influenced by the positional balance shift of the four divided detection regions of the optical pickup. This consequently causes a seek failure on the optical disc of the optical pickup or a vibration phenomenon in the optical pickup, which in turn poses a problem of defocusing during seek.